The bleaching of glyceride oils and fats (collectively "oils" herein) often is done with a clay such as an acid clay. The clay becomes spent, i.e., substantially incapable of further economic sorption of color bodies or of other economic use. (Such clay also is used to bleach fatty acids or as a catalyst to assist in polymerizing unsaturated fatty acids to make "dimer" acids and higher polymers.) Frequently such spent clay is mixed with some activated carbon, and the presence of such activated carbon can be tolerated by the instant process.
Usually the spent clay is filtered from glyceride oils in the process of their refining, yielding a cake that often has about 20-55% oil content (mainly glyceride esters) and the balance essentially clay solids. Disposal of such cake as landfill often conflicts with environmental considerations because of oily drainage and/or undesirable degradation or infestation of the oil present. Such perfunctory disposal can cost more than three dollars a cubic yard for hauling, and it also represents a waste of recoverable glyceride esters and/or fatty acids of fair value.
The instant process allows one to adjust the washing operations towards cleansing the clay thoroughly, regardless of how incomplete the initial clay washing turns out to be. Furthermore, the total processing time can be made unexpectedly short because no compromise need be made between conditions for obtaining substantial and rapid oil collection on the one hand and those for virtually complete and rapid clay cleaning on the other. Additionally, the process permits an autogenous (super-atmospheric) pressure saponification operation for obtaining controlled molecular rearrangement as well as recovery of free fatty acids to maximize their utility or value; such conditions are clearly incompatible with a collection of glyceride ester, thus representing a totally unexpected combination of operations in cleansing saponifiable oil from spent clay.
The desirability of recovering as esters various saponifiable oils (which are mostly glyceride esters) from spent bleaching clay has been evident for many years. The U.S. patent art reflects a variety of organic solvent extraction treatments and a variety of aqueous treatments for this purpose. The latter proposals include the use of various surface tension-reducing agents in the water to assist the oil separation or "washing" treatment of the spent clay, for example, added synthetic detergents, sodium aluminate, sodium stannate, sodium fluoride, water-soluble coagulant salts and added or in situ-produced sodium soaps of fatty acids. Of all these treatments, three heretofore patented proposals and a well-known reference text are believed to be the ones most pertinent to the instant process.
U.S. Pat. No. 1,828,035 shows spent clay washing with aqueous saline solution containing caustic soda or caustic potash sufficient for neutralizing the free fatty acids present, but insufficient for saponifying any appreciable proportion of the glycerides present. Such saponification, it is stated, then would cause the emulsification of glycerides and create difficulty in separating same from the aqueous phase. The free fatty acids are completely neutralized at pH about 8. Sodium chloride is stated to be replaceable by sodium sulfate or other salts tending to repress emulsification of fatty oils in the aqueous liquid.
U.S. Pat. No. 2,706,201, to the extent it is directed to separating saponifiable oils from spent clay, shows adding sodium carbonate to a boiling mixture of clay and water "until a distinct alkalinity to phenolphthalein persists." Such pH indication starts at 8.3, although this would take a highly trained eye to notice it, and the red color is reasonably evident to most users by pH 9. In augmentation of this treatment the patent also suggests adding a synthetic detergent or soap. Many such saponifiable oils contain a very small proportion of free fatty acids; their neutralization to form sodium soaps under these conditions is unavoidable. The patent goes on to point out that where the required approximately 1% of soap cannot be formed in this way, it can be obtained by deliberately saponifying some of the ester with a bit of caustic soda.
While it is not absolutely clear from these references just how much of the glyceride esters present on the spent clay are recovered as glycerides nor just how clean the clay becomes from such treatment, it should be fairly evident that some part of the glyceride esters present are recoverable. However, it must be realized also that various clays can hold onto minor but objectionable oily fractions with differing degrees of tenacity and, furthermore, that the practices shown in these two patents will not necessarily cleanse the clay to the degree necessary for meeting even moderately demanding environmental standards. The latter observation will be evident from the Examples 1-3 of this application where the highest glyceride oil recovery from spent clay occurred at a much higher pH than that at which fatty acids alone are neutralized without substantial saponification of the esters and even much above that pH where the phenolphthalein color is quite evident (pH 9). With use of even a higher pH (10.65) than these, these Examples show that about 65% of the original oil is still retained with the clay in the most favorable case for oil recovery, and at lower pH's a great deal more. All of such residual oil contents could be distinctly objectionable for using the clay as sanitary landfill.
The well-known text Bailey's Industrial Oil and Fat Products, Daniel Swern, Ed., Interscience Publishers, 1964, New York, N.Y., pages 785-786, teaches the saponification of the oil on such clay with aqueous caustic soda at a temperature near the boiling point of the aqueous dispersion. Substantially complete saponification of the oil present also is described in U.S. Pat. No. 1,763,167 wherein aqueous soda ash instead of caustic soda is used at temperatures of 185-210.degree. F. (85-99.degree. C.). (pH in excess of 11 can be attained with even very dilute soda ash in water, and such pH at elevated temperature can cause virtually complete saponification of glyceride ester.) U.S. Pat. No. 1,763,167 shows removing all but roughly 2% oil content from spent clay.
The instant process stages the washing treatment, and it includes the following advantages over prior art suggestions: it permits the recovery of appreciable glyceride esters as such from the spent clay; a more drastically alkaline stage follows a milder one to exert its more powerful cleansing action on just the remaining and often minor proportion of oily residue left; it provides highly cleansed clay suitable for sanitary landfill with a minimum of alkali reactant; and the process is uniquely adaptable to current vegetable oil processing operations which in itself adds special merit that will become evident hereinafter.
Nearly every large vegetable oil processing plant accumulates from its alkali refining of such oils a large quantity of what is called "soapstock." This soapstock can be about half water, contains an appreciable fraction of sodium soaps of fatty acids, some glyceride esters, and usually various minor proportions of unsaponifiable organic material. Usually the glyceride oil refiner either sells it for saponification to convert the esters and soaps into free fatty acids, or he does the necessary saponifying and acidulating right in his own plant to make such free fatty acids. Thus, one of the prime advantages of the instant invention is that it can combine efficiently and effectively: (a) the cleansing of oily material from the spent clay to an extremely high degree for its sanitary disposal, with (b) the recovery of some glyceride ester as such, (c) the preparation of soaps of fatty acids for recovery of the latter, and (d) the utilization of such refiner's soapstock disposal scheme or saponification process to handle process purges or whole separated aqueous bodies from the instant spent clay deoiling operation. Besides that, the process allows for a tolerance in operation in many instances where more or less glyceride ester can be recovered relative to fatty acid sold or recovered, thus giving the plant chemical operators fair latitude while practically precluding insufficient cleansing of the clay. The process is adaptable to processing spent bleaching clays from a wide variety of glyceride oils and fats, although clays from the bleaching glycerides of C.sub.16-18 and higher fatty acids appears to be the best for processing according to the precepts of this invention.